1. Field of Invention
This invention is directed to systems and methods for regulating temperature in fluid ejection devices.
2. Description of Related Art
Inkjet printing devices have gained prominence in printing as result of their capabilities in performing quality, economical color and monochromatic printing. Inkjet printing devices include, but are not limited to, piezoelectric inkjet printing devices and thermal inkjet printing devices. Piezoelectric inkjet devices eject ink from a nozzle by mechanically generating pressure to deform an ink chamber. Thermal inkjet devices eject ink by energizing a heater element to vaporize ink.
In such inkjet printing devices, a printhead, which acts to eject ink onto a recording medium, is comprised of at least one fluid ejecting die module, a substrate to which the die module is bonded, an ink manifold which brings ink to the die module, and electrical interconnection means for enabling the transfer of electrical signals to and from the printhead. The die module typically contains many individual drop ejecting elements, such as piezoelectric actuators or thermal ink jet heaters. In many types of inkjet printheads there is only one die module in the printhead. In other types of inkjet printheads, where it is desired to enable faster printing throughput than can be achieved using a single die module, several die modules are contained within the printhead. Because the fluid ejection process is dependent on the local temperature near the drop ejecting elements, it is important that the temperature be somewhat uniform in the various regions containing drop ejecting elements, whether within a single die module, or among several die modules. In addition, because fluid ejection can become unstable if the temperature gets too high or too low, it is important to keep the temperature within a certain range.
The die module in a thermal inkjet printhead generates significant amounts of residual heat as ink is ejected by heating the ink to the point of vaporization. This residual heat will change the performance, and ultimately the ejection quality, if the excess heat remains within the printhead. Changes in printhead performance are usually manifested by a change in the drop size, firing sequence, or other related ejection metrics. Such ejection metrics desirably stay within a controllable range for acceptable ejection quality. During lengthy operation or heavy coverage ejection, the temperature of the printhead can exceed an allowable temperature limit. Once the temperature limit is exceeded, a slow down or cool down period is normally used to maintain the ejection quality. In addition to self-heating of the printhead, various ambient conditions may make it advantageous to regulate the temperature of an inkjet printhead or other fluid ejection device.
A variety of devices and methods are conventionally used to dissipate heat in an inkjet printhead. Many inkjet printing devices improve throughput by improving thermal performance. One technique to improve printhead performance is to divert excess heat into the ink being ejected. As the hot ink is ejected from the printhead during printing, some amount of printhead cooling occurs as a result. During lengthy operation or heavy coverage ejection, this technique is also susceptible to temperatures in the printhead exceeding an allowable temperature.
Another technique is to attach the die module to a substrate having heat sinking properties. Such substrates store heat and/or conduct heat away from the printhead. Typically, such substrates are made from copper, aluminum or other materials having high thermal conductivity to remove heat from the printhead. U.S. patent application Ser. No. 10/600,507, which is incorporated herein by reference in its entirety, discloses various exemplary embodiments of such substrates molded from a polymer mixed with at least one thermally-conductive filler material.
Thermally conductive substrates, however, add additional weight, size, cost and/or energy usage to the printhead. Each of these becomes disadvantageous when in thermally conductive substrates attached to die modules that are translated past a receiving medium. Moreover, thermally conductive substrates typically dissipate heat via convection, and are inherently ineffective due to their small size.
FIG. 1 is a schematic of a known inkjet printing system 100 showing one method by which ink is conventionally provided to a printhead 130. The system 100 includes a remote ink reservoir 110 and a printhead 130. The printhead is comprised of at least one die module 132, which is bonded to substrate 133, and an ink manifold 131 which brings ink via first fluid communication path 134 to the die module 132. Other components of the printhead 130, such as electrical interconnection means, are not shown. Typically, the remote ink reservoir 110 contains a much larger volume of ink than the ink manifold 131. The remote ink reservoir 110 can be 10 to 1000 times as large as the ink manifold 131. In the case of a scanning type of printhead, this type of ink supply configuration allows the mass of the moving printhead to remain small so that accelerations and decelerations of the scanning printhead do not exert unacceptably large forces on the printer. For either a scanning type of printhead or a stationary type of printhead, there is also typically not enough space near the printhead to store the entire supply of ink. The ink reservoir 110 and the ink manifold 131 are connected by a second fluid communication path 150. The second fluid communication path 150 allows ink stored in the ink reservoir 110 to be provided to the ink manifold 131. The ink is then supplied to the die module 132 as necessary to effect ejection of the ink from the printhead 130 onto a recording medium.
Inkjet printing systems, such as shown in FIG. 1, are limited in their ability to dissipate heat. Such systems are limited because heat can only be dissipated via contact between the printhead and the thermally conductive substrate, and through ejection of ink during printing operations.